In my above-identified application Ser. No. 406,345, I disclose roadway sensors in which a piezoelectric sensor is carried within an envelope and a linear weighting member is substantially coextensive with the sensor to maintain the sensor on the roadway despite being traversed by heavily loaded trailer trucks traveling at high speeds generating trailing air turbulences having the effect of sweeping the roadway. The weight member is uniformly distributed along the roadway portions of the sensor strip to maintain the sensor on the roadway and substantially immune to air effects generated by vehicular traffic on the roadway, such as a loaded truck trailer traveling at high speeds. Preferably the weight is a flat malleable metal such as a lead strip having a weight of about one pound per linear foot. For lower speed roadways and/or roadways restricted to smaller or lighter vehicles (which cause less air flow effects) a lower distributed weight can be used, for example, one-half pound per linear foot of sensor.
Since the weight member, in a preferred embodiment, is a strip of lead, preferably flat for low vertical profile purposes, and since roadways are not flat, e.g., road surfaces wear and wide grooves develop over time and create a cavity or groove where the wheels travel, the flat malleable lead strip adapts to such cavities and undulations and curvatures in the roadway and hugs the surface so that the sensor does not bounce and oscillate for a long period of time as would be the case if the sensor were mounted on a rigid member, such as a rigid steel strip. During the period of time when the rigid steel strip is vibrating, the sensor output signal would mask legitimate pulses from a tire of a closely spaced axle, for example.
The weighted sensor systems disclosed in my above-identified applications monitors a variable number of lanes simultaneously, preferably from one to six lanes or more. In a worst case situation, six lanes, this would require 12 feet times 6 equals 72 feet, plus 8 feet for the shoulder or an 80 foot length. With a rigid steel base member, this would be difficult to install in the field. In my above-identified applications, I teach that the sensor is maintained essentially motionless when a vehicle is traversing the sensor if the linear weight is sufficient to avoid movement effects due to air flow generated by the moving vehicle. Large trucks with wide square backs are one of the worst. As discussed above, experience has demonstrated that a weight of one pound per foot is good for traffic up to speeds of about 85 mph. In order to achieve this weight, a 2" wide piece of lead 3/32" thick is glued to a five ounce per foot elastomeric or rubber carrier. The completed assembly is approximately one pound per foot. In contrast, because the specific gravity of steel is approximately 7.89, and that of lead is 11.35, the steel would need to be thirty percent thicker than lead (0.093" vs. 0.121"). This thicker steel would make the handling of a long sensor very difficult and cause the strip to vibrate as discussed above and, therefore, an integral steel strip is not able to perform according to the invention.
The present invention adapts this distributed weight concept of my earlier above-identified applications and, in addition, provides a unique sensor carrier which is both coilable and reusable and refurbishable and adaptable to wide varieties of roadway conditions and is easily adaptable to multiple lane roadways.
According to one embodiment of the invention, a carrier made of an extruded or molded roadway rubber (such as Neoprene, etc.) is provided with one or more sensor carrying grooves and, depending on the number of lanes of roadway traffic to be sensed, a plurality of parallel signal conductor carrying grooves. In a preferred embodiment, the sensor carrier has an upper surface which is ramped and a lower surface in which are formed a cavity or recess for receiving the weight and has a cross-section corresponding to the cross-section of the weight. In this embodiment, the top surface of the cavity is provided with sensor receiving grooves and coaxial signal conductor receiving grooves. In a preferred embodiment, the sensor receiving grooves are lipped whereas the coaxial signal conductor receiving grooves are deeper and U-shaped. A flat weight member is secured in the cavity or recess to the lower surface of the carrier (the top surface of the cavity), preferably by an adhesive such as a double-faced tape. The weight is lead, it is preferably provided with a protective overcoat, such as a flexible polymer coating. A layer of nylon reinforced tape is adhered to the bottom surface of the lead strip which makes contact with the road. An important feature of this assembly is that it is able to adapt to curvature and undulations in the roadway and hug the roadway surface in such a way as to eliminate or minimize extraneous signals caused by the sensor being bounced up and down on the roadway by traffic and/or by aerodynamic effects caused by high speed heavy vehicular traffic thereover.
In some cases, coaxial cable can have a piezoelectric effect, and enlarged coaxial cable grooves disclosed herein mitigate this effect.
In further embodiments, to protect the lead strip the cavity is formed as a totally enclosed passage or aperture in the elastomeric carrier and the lead weight, with a protective polymer plastic overcoat and lubricated with a solid lubricant such as Teflon.TM. is inserted into the cavity. As a further embodiment, the underside of the elastomeric carrier can be slit to enable easy insertion of the lead strip in the enclosed cavity.
The carrier/sensor of the present invention has the following benefits over road tubes and other prior art systems:
(a) Conforms to and hugs roadway surface; PA1 (b) Simultaneous multiple individual lane sensing; PA1 (c) Adjustable, ideal for driveway and turning movement studies; PA1 (d) Excellent for speed, volume and classification sensing; PA1 (e) Eliminates the need for a mechanical air switch; PA1 (f) Eliminates recording failures due to pin hole air leaks in road tube; PA1 (g) Eliminates recording failures due to water in the road tube; PA1 (h) Eliminates recording failures due to nails coming loose due to high tension stretch on the road tube; PA1 (i) Greater accuracy; PA1 (j) Safe, quick installation; and PA1 (k) Rugged, long lasting and reusable.
Traffic engineers throughout the world have been seriously hampered in their efforts to perform volume, speed and classification studies when road tubes are utilized for input to their traffic recorders. The portable over the pavement piezoelectric polymer sensor and method disclosed herein offers the traffic engineer a means of generating electrical impulses when the vehicle's axle traverses the sensor assembly. This invention provides the traffic engineer volume, speed and classification data which would be virtually impossible to record with existing road tubes. Using this method also enables the traffic engineer to field install the piezoelectric sensor assembly in inclement weather on the roadway and to adjust its active detection area to the requirements of the data capture application. An important advantage of this coilable, removable and reusable sensor is its safety and ease of installation. There is no need to dart in and out of traffic attaching clamps or adhesive tape at different points in order to stabilize the sensor. Its rugged construction and wide temperature operating range make it ideal for slow and fast moving traffic in extremely wet or dry weather conditions.